I cannot fathom how you could turn a side-scrolling game into an MMO game.

Remember that MMO has a meaning. It was a term for games with an enormous online population, these days meaning about 100,000 concurrent people.

While a lot of kids playing with their toy systems will call any game that plays online an MMO, that is not what the term was referring to. Richard Garriott used the term to refer to the type of architecture required for the modern sprawling high-player games with hundreds of thousands of active players. These systems are far beyond a dedicated system, far beyond something a moderately large team can generate. They are truly a massive undertaking, costing several tens of millions of dollars annually in infrastructure alone. The infrastructure and day-to-day operations costs more than the development cost for most games.

I know many youth these days equate "MMO" with "persistent online world", but they are radically different. People can (and do) build a persistent online world infrastructure in a few hours. The networking forum FAQ has one written and and playable in under a week. Persistent online world, but a maximum player capacity of perhaps a hundred or two simultaneous players.

These days they have a notation of the number of concurrent players. Five thousand concurrent players, a C5K system is achievable with simple architecture and isn't massively multiplayer. Ten thousand (C10K) or fifty thousand (C50K) is achievable on a budget of ten or twenty million dollars, and you could probably host that with Amazon or Google or similar architecture that scales dynamically, you could reasonably handle that with a cluster of 10 or 20 servers pretty handily. But even that isn't massively multiplayer, that's still just a regular online world. When you reach larger scales, C100K, C250K, that is reaching a massive architecture. A quarter million concurrent players means an enormous infrastructure spread across the globe.

I cannot fathom tens of thousands of people, or even thousands of people, in a side scroller simultaneously.

I can imagine such a system with hundreds of simultaneous people, but that is painful in my mind

Maybe you've got a design that is different from the side scroller style I picture.

So if you want to build an online multiplayer side scroller world, that certainly is achievable. But I cannot fathom an MMO side scroller no matter how I try. The "massive" modifier is a big thing.

Yes, each realm has its own capacity. During WoW's peak most Realms (the individual clusters of the game) had between 8000 to 15000 people. Transferring characters between realms took a request, a fee, and about three days. As they've declined Realms needed to be merged.

This stands in contrast to many other online worlds with shared worlds, with the opposite extreme of EVE Online being one single shared game with everybody.

Collectively WoW's infrastructure peaked at about 12 million subscribers, with over a million connected at any given time on servers and realms scattered around the globe. Even though Realms were isolated, the infrastructure is still quite significant.

If I remember correctly - and it's been a long time, so don't quote me on this - each realm is split into continents, and each continent was covered by a small number of servers each covering a geographically contiguous area (possibly one zone, possibly several). When moving from one zone to another (or at least a zone on a different server), there would be a server hand-off where you connect to the new one and disconnect from the old. As such, each server might only be handling a few hundred players - possibly even fewer once the instanced dungeon stuff came in.

I cannot fathom tens of thousands of people, or even thousands of people, in a side scroller simultaneously.